1. Field of the Invention
The present invention relates to a laser desorption/ionization mass spectrometry method, a sample supporting substrate used in laser desorption/ionization mass spectrometry, and a method of testing a material for the sample supporting substrate.
2. Description of the Related Art
Mass spectrometry using a laser for ionization is used for, for example, measuring the molecular weight or atomic weight of a sample or analyzing its molecular structure or components in such fields as the chemical industry, clinical medicine, and biotechnology. Mass spectrometry includes LDI-MS (Laser Desorption/Ionization Mass Spectrometry), which includes MALDI-MS (Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry).
According to MALDI-MS, ions are generated by exposing a sample solution dropped on a sample plate to laser light after evaporating its solvent. The generated ions are spatially or temporally separated in accordance with their mass-to-charge ratios (m/z) in an electric field or magnetic field, and electrical signals reflecting their amounts are measured.
MALDI-MS is regarded as excellent as a highly sensitive spectrometry technique but as limited in usefulness because of difficulty in ensuring data reproducibility due to frequent variations in the amount of ions generated at each laser pulse.
This is partly because a sample on the substrate tends to have a small thickness in order to keep the electric field uniform where ions are generated and the substrate surface often contains minute unevenness in order to have the sample evenly adhere to the substrate surface. Therefore, laser light often reaches the substrate surface through the sample to cause ions to be generated from the interface between the substrate and the sample, so as to cause a mixture of the generated ions with ions from the sample.
It is believed that the amount of ions observed greatly varies because of the mixture of ions generated from two different environments and variations in the mixture ratio due to the thickness distribution of the sample and the evaporation of the sample by exposure to laser light. As a result, there is an increase in undesirable ions other than those derived from the sample, that is, background noise.
Known techniques for reducing background noise are as follows.
(a) Japanese Laid-Open Patent Application No. 5-062643 shows a time-of-flight mass spectrometer having an electrostatic shutter and a time-of-flight mass spectrometry method using the same, where undesirable ions are removed by spatially and temporally selecting those to be introduced into an analyzer from generated ions. Further, Japanese Laid-Open Patent Application No. 2001-057174 shows a magnetic sector mass spectrometer that removes undesirable ions in the same manner.
(b) Japanese Laid-Open Patent Application No. 2004-158005 shows a method of creating a database and a database for identifying a polymorphic genetic marker, where signals of undesirable ions, that is, noise, is removed by performing mathematical processing on a measured spectrum.
(c) Japanese Laid-Open Patent Applications No. 2006-201042 and No. 2006-329977 each show an ionization substrate for laser desorption/ionization mass spectrometry and a laser desorption/ionization mass spectrometer, where a material that is less likely to cause generation of undesirable ions is used for the substrate.
However, such a method that selects ions to be introduced into the analyzer as shown above in (a) is less effective in removing the noise of background ions spatially or temporally close to necessary ions.
Further, such a method that performs mathematical processing on an already measured spectrum as shown above in (b) is limited in removing background noise because the method cannot help being applied to a spectrum that has already contained noise since no consideration is given to whether laser light has reached the substrate.
Further, such a method that reduces the amount of ions to be generated from a substrate material as shown above in (c) has the difficulty of not being able to give consideration to whether laser light has reached the substrate because once the laser light reaches the substrate, background ions may be generated from a sample at the interface between the sample and the substrate, thus making it impossible to observe ions derived from the substrate material.